The present disclosure relates generally to switching mode power supply, and more particularly to switching mode power supplies with primary side control (PSC).
Almost all electronic appliances require power supplies. A power supply converts for example an alternating-current (AC) voltage of a power grid into a power source with specific ratings demanded by the core circuit of an electronic appliance. Among all kind of power supplies, switching mode power supplies are known to be compact in size and efficient in power conversion, and therefore they are broadly adopted by power supply manufactures. Switching mode power supplies normally use pulse width modulation (PWM) technology to control power conversion.
In order to avoid overstress damage or explosion happening to electronic appliances because of lightning strike to a power grid, official regulations in many countries mandate isolation between primary and second sides in power supplies, meaning there is no direct-current (DC) allowed to flow between the primary and second sides. The voltage levels at the primary side are all in reference to the input ground of the power grid, and those at the secondary side to a floating ground.
Switching mode power supplies could be controlled by providing a PWM signal to manipulate a power switch at a primary side, so as to control the power conversion from the primary side to a secondary side and make an output voltage of a power supply meet specific ratings. For example, the output voltage should be regulated to be within a tolerance range centering on 5 volt.
Primary side control (PSC) is known to use only information at a primary side for regulating an output voltage at a secondary side. A power controller at the primary side senses a reflective voltage on a winding of a transformer to detect indirectly the output voltage, so as to control power conversion. Secondary side control (SSC) nevertheless has circuits at a secondary side to detect directly an output voltage, and passes resulted information to a power controller at a primary side through an isolation device such as a photo coupler.
FIG. 1 demonstrates a switching mode power supply 10 with PSC. Power controller 12 has a current sense node CS, operating voltage node VCC, drive node DRV, feedback node FB, and input ground node GND. A transformer comprises primary winding PRM, auxiliary winding AUX and secondary winding SEC, all inductively coupled to one another. Power controller 12 at the primary side indirectly senses the output voltage VOUT at the secondary side during a period of time when the transformer is demagnetizing, by way of the signal path provided by feedback node FB, resisters 14 and 16, auxiliary winding AUX, and secondary winding SEC. Power controller 12 accordingly provides PWM signal SDRV to make the transformer magnetize or demagnetize.
Dynamic load testing tests how a power supply responds when its load changes in a severe and repetitive manner. FIG. 2 shows dynamic load testing to switch mode power supply 10 and the results, including the waveforms of current load ILOAD passing through load 18, PWM signal SDRV, switching frequency fSW, and output voltage VOUT. As shown in FIG. 2, during dynamic load testing, load 18 of switching mode power supply 10 is controlled to jump back and forth between a heavy-load condition and a no-load condition. Switching frequency fSW of PWM signal SDRV becomes high under the heavy-load condition and low under the no-load condition. When load 18 changes, output voltage VOUT due to limited bandwidth of the feedback loop that tries to keep it at target voltage VTAR, drifts away temporarily, generating overshoot or undershoot. The less overshoot and undershoot the better regulation ability of s power supply, and the better dynamic load response.
Optimization of the switching frequency fSW under a no-load condition is tricky though. To enjoy less switching loss and higher power conversion efficiency, the switching frequency fSW of PWM signal SDRV needs to be low under a no-load condition, as indicated in FIG. 2. The less switching frequency fSW benefits a PSC switching mode power supply in power conversion efficiency, but possibly at the expenses of dynamic load response. As power controller 12 with PSC in FIG. 1 senses output voltage VOUT only during demagnetizing of the transformer, and is blind to any change in output voltage VOUT before the next time of demagnetizing, a low switching frequency fSW for a no-load condition could cause serious undershoot if load 18 happens to become heavy right after the demagnetizing of the transformer, rendering poor dynamic load response.